Infection occurs when the microorganisms enters the host, is able to multiply and cause damage to the tissues of the host.

The chain of infection involves a causative agent, a reservoir of infection, a portal of exit, a mode of transmission, a portal of entry and a susceptible host. There are specific features at each part of the chain and interactions between parts of the chain determine infection.

Exposure to pathogenic organisms does not always produce disease:
• organism may not be able to divide/grow in site of infection
• needs to reach a specific receptor site
• organism may be destroyed or inhibited by bodies defence mechanism – mechanical barriers, lysozymes, phagocytes, antibodies
• normal flora can inhibit growth

Virulence (the capacity of an organism to cause disease) = infectivity + invasiveness + toxigenicity

Susceptible host:
• important link in chain of infection
• intrinsic risk factors include extremes of age (young and old); socioeconomic status; occupation; immune status
• extrinsic risk factors – health care professionals (eg handwashing etc), invasive procedures,

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